Watersong (The God War's Chosen, #1) by Ryan W. Mueller

When a mysterious assassin attacks her father, Princess Firia tracks the man with the magic of her Watersong. But she's too late. In her grief, Firia supports her mother's ill-fated war for revenge. Now, due to Firia's failures, rifle-wielding invaders patrol her tropical island nation, executing anyone who openly worships the goddess Shara.

Firia joins the resistance, secretly pushing her Watersong to its limits. She will not fail her people again. She will not allow her goddess to weaken and die as faith in her declines. If Firia is discovered, she'll face execution herself. But she can manipulate water into a weapon, shield herself from bullets, and even capture a perfect and unalterable water memory of an event. If she can't save her people, then no one can.

With her rare talents, she spies on traitors within the resistance and discovers the truth. The assassin who killed her father is the power-hungry god Theos, and Theos is manipulating the invaders into doing his bidding. Firia has one chance to reveal her water memory and show the invaders their true leader. But Theos knows about her and will stop at nothing to see her dead.

Watersong is an epic gunpowder fantasy. It also tells the story of Firia's brother Sareli, who is left for dead as a prisoner of war and must fight in a brutal arena to raise an army for his people, and the story of Rathin, the commander of the invading army, who struggles between following his duty and following his conscience.